The continuous monitoring of vacuum pressure is critical for machinery operating under high vacuum and is of particular concern in the operation of absorption cold generator refrigeration systems such as employed in industrial and commercial air conditioning equipment. The typical absorption air conditioning system is charged with a lithium bromide solution which acts as an absorbent and employs water as the refrigerant. The lithium bromide has a strong affinity for water vapor. The refrigerant is supplied to an evaporator chamber and is sprayed over a bundle of tubes through which water passes and is chilled. The chilling takes place as a result of evaporation of water on the tube surface as it is absorbed by the lithium bromide. The heat needed for evaporation is taken from the water within the tubes thus lowering the temperature and producing the chilling effect. The weakened solution of lithium bromide is then pumped into a concentrator where the refrigerant vapor and absorbent solution is heated to release the refrigerant. The reconcentrated solution is then returned to the absorber and the water vapor is directed to a condensor where the refrigerant condenses into a liquid and returned to the evaporator thus completing the cycle.
The evaporator/absorption chamber is maintained at a low pressure, for example 0.25 in. mercury, (Hg), so that the boiling temperature of the refrigerant is sufficiently low to produce the desired chilled water temperature. It is also necessary to maintain a pressure within the evaporator/absorption chamber at or near absolute 0 in. Hg for operating the refrigeration system at maximum capacity.
A small differential in vacuum pressure thus directly effects the capacity and efficiency of the system. Furthermore, a precipitous low in vacuum pressure can result in crystallization of the lithium bromide solution.
Although there are control devices which have been designed to monitor refrigeration systems such as typically shown in U.S. Pat. No. 3,707,851, these devices do not automatically monitor vacuum pressure.
For precision measurement of vacuum pressure approximating absolute zero, a mercury tube manometer is preferred over a bourdon gauge. These instruments are conventionally read by visual observation of the liquid meniscus. In order to affect automatic pressure monitoring, indirect or remote reading is required. A previous attempt for remote reading included the use of a radioactive material which was floated on the mercury, as shown in U.S. Pat. No. 2,714,168. A disadvantage of that arrangement, especially as applied to vacuum pressure measurement, was that radioactivity could contaminate the system. Another device utilized an electronically recording manometer, as shown in U.S. Pat. No. 4,631,960 for providing atmosphere pressure readings. Such a device would not be suitable for use in a vacuum pressure environment.